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In Texas the three day rule applies to buying a house, door to door sales, and magazine subscriptions due to federal law. However, there is no automatic three day rule for cable, or dish service.
In Texas the right of rescission law applies only to very specific situations. The three day right of rescission law applies to sales made at facilities other than the seller's place of business. Texas law on 3 day rescission is set out in Chapter 601 of the Texas Business & Commerce Code.
However, check your contract, if there is a three day right in the contract then, you can still cancel with written notice. The contract will detail how to give the notice.
Also, if DISH has committed a deceptive trade practice or otherwise by misrepresenting to you that the service would work, and it does not, then you may have a right to rescind the deal on that basis. If those are the facts, you should write a letter by certified mail to the seller detailing how the seller deceived you or has misrepresented something to you.
So if you feel that the deception or misrepresentation is significant enough, you can notify the seller in your letter that you are rescinding the transaction.
I hope this helps with your question.